The present invention relates to a process for the production of mixtures of diisocyanates and polyisocyanates from the diphenylmethane series with high contents of 4,4′- and 2,4′-methylenediphenyl diisocyanate and to the use of such mixtures for the production of polymers.
It is known that diisocyanates and polyisocyanates from the diphenylmethane series (MDI) are produced by phosgenating the corresponding diamines and polyamines from the diphenylmethane series (MDA). The diamines and polyamines from the diphenylmethane series are themselves produced by condensation of aniline and formaldehyde. By phosgenating the diamines from the diphenylmethane series the corresponding diisocyanates 2,2′-MDI, 2,4′-MDI and 4,4′-MDI which people skilled in the art describe as binuclear MDI (diisocyanates from the diphenylmethane series) are obtained. During the condensation of aniline and formaldehyde, the binuclear MDA (methylenediphenyl diamine MDA) also reacts further with formaldehyde and aniline, however, to produce higher-nuclear MDA grades, which after phosgenation represent the polynuclear content in the polymeric MDI (polyisocyanates from the diphenylmethane series). For many practical product applications it is preferable to obtain a high proportion of binuclear MDI. According to the current prior art this can be achieved in two different ways.    1. Aniline is used in large excess in the acid-catalyzed condensation of aniline and formaldehyde, the excess aniline is separated off from the reaction mixture and recycled. The large excess of aniline during the condensation produces an MDA mixture with a high proportion of binuclear MDA. The higher the ratio of aniline/formaldehyde used in the reaction, the higher the binuclear content contained in the reaction product. The ratio of 2,4′-MDA to 4,4′-MDA can also be influenced by the concentration of acid catalyst. High catalyst concentrations favor 4,4′-MDA. Low concentrations favor 2,4′-MDA. (See, e.g., EP-158059-B1 and EP-3303-B1.) Such MDA grades are referred to below as polymer A or polymer B grades for MDI-compositions comprising low 2,4′-isomer content or high 2,4′-isomer content, respectively. The desired MDI composition can be obtained by phosgenation of the MDA grades selectively produced in advance.    2. Polymeric MDA is produced in the conventional way from aniline and formaldehyde using an acid catalyst. The polymeric MDA is phosgenated and broken down by distillation into a highly monomer-rich and a polymer-rich fraction. The polymeric MDI fraction can be used as a commercial polymeric MDI product. The monomeric MDI fraction is broken down according to the prior art by distillation or by crystallization into the isomers 4,4′-MDI and a mixture of approximately 50% 2,4′-MDI and 50% 4,4′-MDI. The two monomeric products are sold or processed further to produce mixed products with polymeric MDI having high contents of binuclear MDI and in the correct isomer ratio.
In both cases, the investment and energy consumption involved mean that the cost of producing MDI mixtures having a high proportion of binuclear MDI is considerable.
In the first case, in the condensation of aniline and formaldehyde large excesses of aniline must be circulated, separated from the reaction mixture by distillation and then recycled.
In the second case, the condensation of aniline and formaldehyde with lower excesses of aniline produces considerable amounts of higher-nuclear MDA grades in addition to the desired diamines, which are then phosgenated together with the diamines. In order to obtain the diisocyanates from the mixture of diisocyanates and polyisocyanates (binuclear MDI and higher-nuclear MDI grades), the diisocyanates have to be distilled out of the mixture of diisocyanates and polyisocyanates (monomer/polymer separation). After this monomer separation, the isomers must be separated from one another by distillation and/or crystallization, involving the use of large amounts of equipment and high energy consumption. The laboriously separated isomers are then mixed together again in high-monomer-content finished products.
The crude monomer distillate (crude monomer, consisting substantially of diisocyanate) obtained by monomer/polymer separation in the process according to the prior art still contains undesirably high concentrations of secondary components, however. For example, this crude MDI monomer mixture contains many times the maximum concentration of 50 ppm monochlorobenzene and a maximum of 20 ppm phenyl isocyanate, preferably a maximum of 20 ppm monochlorobenzene and a maximum of 10 ppm phenyl isocyanate commonly demanded by polyurethane manufacturers. In addition, conventional crude MDI monomer mixture contains a considerable amount of uretdione (dimeric para-MDI), which in larger concentrations leads to product turbidity and solids precipitation in the MDI. The diisocyanates distilled off in the monomer/polymer separation must therefore be freed from secondary components in subsequent distillation stages involving considerable cost and effort.
Special MDI grades with specified contents of isomers, in other words 2,4′-MDI, 2,2′-MDI and 4,4′-MDI, are traded commercially. According to the prior art the compositions corresponding to the commercially available MDI grades with the low secondary component contents are therefore produced by complex, multistage isomer separation by means of distillation and/or crystallization. The isocyanate composition required for a specific application is then produced by blending these MDI grades.
For many applications of MDI grades, however, polymer production does not require a very high 4,4′-MDI purity, in other words a very low 2,2′-MDI content, for example, in the feedstocks to be blended. The use of a pure 4,4′-MDI grade with very low 2,2′-MDI contents produced by means of several complex distillation stages for mixing with other MDI grades to produce an isocyanate composition that is suitable for the specific application is therefore unsatisfactory in terms of energy consumption.
The following is also known from the prior art:
The following two works, for example, describe the direct production of mixed MDI products by means of a selective MDA synthesis. Keggenhoff, Maehlmann & Eifler (EP-158059 B1) produced an MDA mixture with a high 4,4′-MDA content, containing approximately 80% 4,4′-MDA and approximately 10% 2,4′-MDA, and a binuclear content of approximately 90%. By contrast, Eifler & Ellendt (EP-3303 B1) were able to produce a high-monomer-content MDA with 88% binuclear content, containing 19% 2,2′-MDA, 36% 2,4′-MDA and 45% 4,4′-MDA. High molar ratios of aniline/formaldehyde of over 8 are needed to produce these products, which means that large amounts of aniline have to be recycled. In addition, high consumptions of HCl (for catalysis) and NaOH (to neutralize the catalyst) are necessary for many of these grades. Products having above all a high 4,4′-MDA content display a very high specific HCl catalyst consumption. The production of high 2,4′-MDI-content, monomer-rich MDA grades in particular leads to a high, often undesirable secondary yield of 2,2′-MDI. High concentrations of 2,2′-MDI are undesirable in many applications because of its low reactivity.
The production of polymeric and monomeric MDI products is generally known in the literature. The production of polymeric MDA with binuclear contents of 46 to 65% can take place, for example, in accordance with DE-2750975 A1 or DE-2517301 A1. The two processes of distillation (DE-3145010 A1) and crystallization (EP-A-482490) that are predominantly used in industry for isomer separation in the MDI monomers are described in detail in the literature. None of the works, however, describes the direct use of the crude monomeric MDI fraction from the monomer/polymer separation as a feedstock source for MDI blends. The people skilled in the art concentrate instead on the most economical production possible of ultrapurified monomeric isomer mixtures containing >98% 4,4′-MDI and a mixture of approximately 50% each of 2,4′- and 4,4′-MDI, which can optionally contain up to 2.5% 2,2′-MDI (M. Stepanski, P. Faessler: “New hybrid process for purification and separation of MDI isomers”, Sulzer Chemtech, Presentation at the Polyurethanes Conference 2002 in Salt Lake City, October/2002).
The production of high-monomer-content MDI mixtures by blending polymeric MDI products and repeatedly distilled monomeric products is a common process in the polyurethane industry. For example, it is used in the production of low-viscosity blended MDI products in U.S. Pat. No. 5,258,417.
The purification of crude, polymeric MDI mixtures has been examined in principle in many works. For example, attempts are made to remove impurities using chemical additives (U.S. Pat. Nos. 3,925,437, U.S. Pat. No. 3,793,362, DE-A-2316028). In DD-A-118,105 and GB-A-1,114,690, solvents are added in an attempt to remove chemically bonded impurities. DD-A-271,820 suggests stripping MDI and TDI (toluene diisocyanate), which causes a marked feedstock decomposition of 0.1 to 10 wt. %, however, and substantially pursues the objective of removing very stubborn, chemically bonded impurities.
U.S. Pat. No. 3,857,871 discloses a stripping process for polymeric MDI which leads to a reduction in acidity and hydrolyzable chlorine in the product. GB-A-1,362,708 describes a process for the purification of polymeric MDI, which admittedly reduces the amount of hydrolyzable chlorine significantly. The product still contains 0.1% solvent, however.